US election: Do presidential debates really matter?

An audience of about 100 million is expected, the largest ever for a US presidential debate [Andrew Harnik/AP]

The anticipation is huge. The audience will be massive and the build-up has been relentless. But the reality is American presidential debates are rarely the "game changers" the pundits and supporters hope they will be.

This time we're told it's different. There are so many new dynamics on the state. There is the man v woman contest. There is the experienced politician against the political neophyte. There is the reality TV star against the candidate who seems to hate any media attention.

The two campaigns have been lowering expectations for weeks.

The Clinton camp has been predicting that Donald Trump will do well because he was the host of a reality TV show that attracted big audiences.

The Trump campaign has been suggesting their candidate has not being doing traditional debate preparations: no big briefing books; no mock debates; no rehearsed answers. It's hard to believe either side is serious.

But if you lower expectations and your candidate does as you expected, you can then spin that it was a "fantastic performance" and the narrative that is created the day after the debate is almost as important as the event itself.

Historians will point to key moments in previous debates that suggest races changed there and then.

There was John Kennedy looking cool and composed in the first ever televised debate, while Richard Nixon who was ill looked sweaty and ill at ease. It's interesting to note that those listening on the radio thought the Republican won that one hands down.

In 1976, President Gerard Ford claimed Poland was not under the Soviet Union's sphere of influence. It is paraded as a massive mistake. But few voters interviewed the next day even recognised the error. And the suggestion that Ford's polling numbers took a dive after that ignores the fact his support was dropping all through the campaign season.

The same is true just four years later in 1980. Ronald Reagan gave an impressive debate performance. Despite concerns about his age (at 69 he was one year younger than Donald Trump is now, and one year older than Hillary Clinton) he was considered to be "calm and in control".

He went on to win the election handsomely. But his opponent President Jimmy Carter was plagued with bad economic numbers, there was a congressional investigation into his brother, and the Iranians were not willing to negotiate the release of hostages held in the US Embassy in Tehran. Before the debate, Reagan had a five-point lead. His performance just gave him a bigger lead.

Just four years ago, everyone thought Mitt Romney won the first presidential debate in Colorado. I was there. He did. President Barack Obama seemed bored, disengaged, and was way too wordy to get his points across.

The Romney campaign headed out of Colorado on a high. Suddenly people were engaged with a candidate many liked but didn't love. At his first event post-debate, people queued for hours to get to see him. I know this because we got stuck in the traffic going to cover it.

But the polls didn't move much. The lead Obama had remained consistent. On the night of the election a Romney campaign supporter told me the Republican had won, the polls were wrong and the momentum generated from the first debate was responsible. The election night party turned from a celebration to a wake.

Most people have already made up their mind on these candidates. The debates may provide a few more snippets of information to those wavering, but people tend to cheer their chosen candidate. And the candidates can largely control how they perform in the debates. They can be prepared, organised and ready to handle any attack.

What will change the face of the election will be those unpredictable events between now and election day - things outside their control - and how they react.

Four years ago, in an excellent analysis in the Washington Monthly, George Washington University political scientist John Sides looked at these moments.

"Scholars who have looked most carefully at the data have found that, when it comes to shifting enough votes to decide the outcome of the election, presidential debates have rarely, if ever, mattered," he concluded.

So why are we expecting an audience of about 100 million, the largest ever for a presidential debate?

Well, people will be watching to see if Clinton keels over at the podium or if Donald Trump throws aside the discipline that has been injected into his campaign and becomes the wild, unfocused, thick-skinned insult machine of the Republican primary debates.

If there is such a thing as a political circus, this is the main show in the ring.